1. Field of the Invention
The present invention relates to information-embedding apparatus, a tamper-detecting apparatus, and arts related thereto. More particularly, it relates to an art of embedding characteristic information as a digital watermark into compressed image data in response to the image compression of a digital image signal, in which the characteristic information is embedded to determine whether or not the compressed image data has been tampered with, and further relates to an art of detecting the embedded characteristic information.
2. Description of the Related Art
In recent years, there has been an increasing demand for a surveillance system which provides crime-proof security. In particular, users eagerly want lengthy recording and high-image quality in a field of a monitoring recorder.
As a result, a digital disc recorder rapidly prevails, which is designed to practice the image compression of digitized image data, and to record such compressed image data as digital information.
The use of commercially available image-processing software results in easy tamper (e.g., editing and processing) of such digital data.
Although the above description exemplifies the monitoring recorder and the digital disc recorder, the present invention is not limited thereto. The present invention is broadly applicable to image-processing arts.
There has been a continuing need to establish a tamper-detecting art that determines whether or not a digital image has been falsified. Electronic authentication is heretofore known as one of the established arts.
FIG. 13 briefly illustrates how prior art electronic authentication is practiced. A data transmitter of FIG. 13 applies a hash function to an original digital image, thereby providing a hash value. The data transmitter compresses the digital image in accordance with the hash value, thereby generating a digest.
The data transmitter encrypts the digest using consignor's private key. The data transmitter transmits both of the original digital image and the encrypted digest to a data receiver through a network.
The data receiver receives the above two different pieces of data through the network. The data receiver compresses the received digital image in accordance with the hash value, thereby producing a first digest. The data receiver decodes the received digest using a consignor's public key, thereby providing a second digest.
The data receiver compares the second digest with the first digest. When the first digest is coincident with the second, then it is determined that no tamper has been made to the digital image. Unless otherwise, a determination is made that the digital image has been tampered with.
The electronic authentication as discussed above requires that the data transmitter sends two different pieces of data (i.e., the original digital image and the encrypted digest) to the data receiver.
When a great number of digital images are present, then a data-managing apparatus operable to properly manage a relationship between the digital image and the digest is virtually essential.
Instead of the data-managing apparatus, there has heretofore been known a digital watermark-based art. A digital watermark is an art of embedding digital information into digital image data in such a manner as to preclude a human eye from perceiving the embedded digital information.
A tamper-detecting method using such a digital watermark art has been proposed. For example, patent reference No. 1 (published Japanese Patent Application Laid-Open No. 10-164549) discloses an image authentication system.
The following briefly discusses the image authentication system with reference to FIG. 14. Target image data “D” (see FIG. 14 (a)) photographed using a digital camera is split into two different image areas “D1”, “D2”. (See FIG. 14 (b)). The image area “D1” is used to generate a hash value. The image area “D2” has the generated hash value “H” embedded therein.
A digest-calculating unit (not shown) calculates hash value “H” on the basis of data contained at the image area “D1”. Hash value “H” is encrypted using private keys that are different for each digital camera. The encrypted hash value “H” is embedded into the image area “D2”.
As shown in FIG. 14 (c), a verification apparatus generates first hash value “H1” on the basis of the data at image area “D1”.
The verification apparatus extracts the embedded hash value “H2” from the image area “D2”, and decodes the extracted second hash value “H2” using a public key.
The verification apparatus compares the second hash value “H2” with the first hash value “H1” to determines whether or not the image data “D” has been falsified.
An art taught by patent reference No. 2 (published Japanese Patent Application Laid-Open No. 11-341268) partially decodes a compressed digital image, and then calculates hash values from all data blocks while a determination is made as to whether or not the hash values are embedded into the digital image in accordance with data block coefficients.
The art according to patent reference No. 2 again partially decodes the compressed digital image, and then replaces one bit of a coefficient at a block in which the hash value is to be embedded and one bit of the hash value with a watermark bit, thereby embedding the watermark bit into the digital image.
However, the art according to patent reference No. 1 executes the above processing in a pixel domain. This means that, when an image data change-involving process (e.g., image encoding) is made in the pixel domain, then first hash value “H1” and second hash value “H2” before such processing totally differ from those after the processing. As a result, it is impossible to practice verification by a comparison of one hash value with another. Consequently, using the art according to patent reference No. 1 it is impossible to detect possible tamper in a digital image that has experienced image compression.
The art according to patent reference No. 2 premises that compressed image data is decoded twice to embed the hash value into the digital image. In addition, the art according to patent reference No. 2 must change variable values that correspond to at least a coefficient. The variable values are provided in a commonly used quantization table. As a result, the art according to patent reference No. 2 involves very complicated processing to detect the possible tamper in the digital image.
Moreover, the arts according to patent references No. 1 and No. 2 make it impossible to distinguish intentional tamper (e.g., replacement of part of an image with another) from an image change due to irreversible image processing that is commonly practiced without malicious intent.